59 
The spermatozoa consist of a minute oval or rather pyramidal body 
with a delicate caudal filament. In each of the vesicles of evolution 
there may be distinctly seen a somewhat elongated nucleus which is 
the body of the spermatozoa, as yet confined in its vesicle of evolu- 
tion, and is plainly derived from the original spherical nucleus of 
the vesicle. 
The ovum, from the earliest period at which I have observed it, 
appears as an opaque yellowish body. From an early stage it may 
be seen to consist of a mass of minute spherical cells filled with a 
yellow fluid, while the whole is enveloped in a delicate vitelline 
membrane. In the young ovum the germinal vesicle may be seen 
as a single large spherical cell included in the midst of the other 
contents, whose opacity, however, makes it necessary to subject the 
ovum to compression before we can bring into view the germinal 
vesicle, which may then be completely isolated as a separate cell on 
the field of the microscope. In the more advanced ovum the ger- 
minal vesicle has entirely disappeared ; but I did not succeed in 
satisfactorily tracing any very distinct segmentation, owing, doubt- 
less, to the unusual opacity of the ovum. The whole ovum becomes 
gradually converted into the embryo. As the time approaches when 
it is to leave the gonophore, we find it capable of changing its form 
by slow contractions, and it soon escapes by the aperture of the 
gonophore, and enters on the external world as a free embryo. 
It is now of a more or less conical form, though continually 
changing its shape by slow contractions. By this time the ectoderm 
and endoderm are both differentiated, and a central cavity has already 
made its appearance; but there is as yet no trace of a mouth; thread- 
cells, the characteristic product of the ectoderm, are copiously deve- 
loped in it, and its surface is clothed with very minute cilia, which, 
however, in all the examples I examined, were so enveloped in a 
mucous investment as to impede their action, and render them 
powerless as organs of locomotion. The embryo creeps about slowly 
upon the sides of the glass jar in which it is confined, avoiding the 
light side of the vessel. 
After enjoying for a period its locomotive stage, the embryo fixes 
itself to the side of the jar by one extremity (the wider ?) which 
then extends itself by means of radiating elongations into a little 
disc of a regular stellate form. From the centre of the free sur- 
face of the disc a cylindrical column now rises perpendicularly, and 
